1. Field of the Disclosure
The disclosure relates to a method for automatically controlling an electrohydraulic pressing tool, in particular for producing pipe connections.
2. Discussion of the Background Art
For producing pipe connections with the aid of press fittings, it is known to use electrohydraulic pressing tools which are operative to generate a plastic deformation of the press fittings. An electrohydraulic pressing tool used for this purpose comprises a hydraulic piston arranged in a hydraulic cylinder. To actuate the pressing tool, i.e. particularly for closing the pressing jaws, fluid will be pumped from a fluid reservoir into the hydraulic cylinder by means of a hydraulic pump. In the process, the hydraulic pump will be driven by an electric motor. Further, the hydraulic cylinder is connected to a valve such as e.g. an overpressure valve. When a limiting pressure is reached in the hydraulic cylinder, said overpressure valve, which normally is of the mechanic type, will be automatically opened, thus allowing the fluid to flow back into the reservoir. In the subsequent pressing process, the hydraulic piston will be pressed back into the base position, e.g. by means of a spring, thereby also causing the fluid to be conveyed back into the reservoir. After termination of the pressing process, the electric motor has to be switched off. The switch-off of the electric motor can be performed by the user, e.g. by releasing the starter.
For effecting an automatic switch-off of the electric motor, it is known to measure the rotary speed of the electric motor. When the overpressure valve is opened, the pressure in the hydraulic cylinder will drop sharply, thus causing the rotary speed of the motor to increase. This increase of the rotary speed can be detected and be used as switch-off signal for the electric motor. In this context, the measurement of the rotary speed requires an external sensor.
Further, from EP 1 230 998, there is known an automatic method for switch-off of the electric motor, wherein the electric current intake of the electric motor is measured. When the overpressure valve is opened, the current will decrease. With the aid of a microprocessor, a drop below a stored electric current value will be detected and, as soon as this predefined fixed electric current value is not reached anymore, the electric motor will be switched off. This method, however, has the disadvantage that the electric current prevailing at the end of the pressing process, i.e. when opening the overpressure valve, is dependent on external influencing factors. The electric current really prevailing at the end of the pressing process will depend e.g. on the charging state of the rechargeable battery, the condition of the motor (condition of the carbon brushes, inner resistances etc.), the temperature of the hydraulic oil, and on the wear of the mechanics of the whole pressing assembly. This can have the result that the electric current prevailing during the opening of the overpressure valve is relatively high, so that the stored switch-off current value will be reached only after a temporally long current drop. As far as the electric current prevailing during the opening of the overpressure valve is smaller than the switch-off current value, the problem exists that no switch-off will take place.
It is an object of the disclosure to provide a method for automatically controlling an electrohydraulic pressing tool, wherein said method shall have an improved reliability.